Dental restorations

ABSTRACT

The invention provides a method for the manufacture of a dental restoration including the steps of forming a model of refractory material of a tooth, and forming, by flame spraying directly on to said refractory model, a base layer of predetermined thickness of said restoration, said refractory model being cooled during the formation of said base layer. The refractory model is subjected to burn out, and optionally hardening heat treatment, and one or more layers of dental porcelain are applied to the base layer following sintering of said base layer.

This is a continuation of International Application PCT/GB98/01921, withan international filing date of Jun. 30, 1998 and now abandoned.

This invention relates to a method for the fabrication of full orpartial dental restorations, including veneers, crowns, inlays, onlays,and bridge structures, hereinafter referred to as dental restorations.

The invention utilises the technique of flame spraying to produce arelatively dense basal layer of a technical ceramic based material whichfirstly acts a permanent form upon which porcelain can be applied inorder to yield ultimate aesthetics of the restoration, and whichsecondly serves to strengthen the restoration and provide a means forbonding the restoration to a prepared tooth in which the contributionmade by the mechanical bonding mechanism is enhanced.

According to the present invention there is provided a method for themanufacture of a dental restoration including the steps of forming amodel of refractory material of a tooth, and forming, by flame sprayingdirectly on to said refractory model, a base layer of technical ceramicbased material of a predetermined thickness of said restoration, saidrefractory model being of high thermal conductivity and cooled duringthe formation of said base layer.

The method may include the further step of firstly subjecting therefractory model to burn out and optionally a hardening heat treatmentat temperatures in the range of 1000-1150° C.

The method may include the addition of silicate based material into theprecursor powder or after the formation of the base layer.

The method may include the further step of sintering said base layer oftechnical ceramic based material and applying to said base layer one ormore layers of dental porcelain.

The method may include the additional step of removing the refractorymodel immediately after forming said base layer and prior to thesintering and application of said layer(s) of dental porcelain, oralternatively removing the refractory model after the sintering stagebut prior to the application of said dental porcelains.

Preferably, the refractory model will be formed by casting in a siliconebased impression of said tooth.

In order that the invention may be more readily understood, anembodiment thereof will now be described, by way of example, referencebeing made to the accompanying drawing, wherein:

FIGS. 1 and 1a are cross-sectional elevations of dental restorationsprepared in accordance with the invention directly on to a high thermalconductivity refractory material model; and

FIGS. 2 and 2a are cross-sectional elevations of the dental restorationsof FIGS. 1 and 1a with the refractory material model removed and shownpositioned relative to a natural tooth.

Referring to the drawings, and firstly to FIG. 1, there is shown asection of a dental restoration indicated generally by reference numeral2. The dental restoration comprises a base layer 4 of technical ceramicmaterial such as Alumina, Zirconia, or Titania, or combinations thereof,and subsequent layers 6, 8, and 10 of dental porcelains which areapplied to the base layer 4 to enhance the aesthetics of therestoration.

In the formation of the restoration, there is firstly formed a model 12of a prepared tooth. The model 12 is composed of a refractory materialwhich is an ‘active’ eliminable material and which comprises essentiallyof an admixture of Silica, Silicon Carbide, Magnesium Oxide, MonoAmmonium Phosphate, and Zircon or Aluminium Oxide in predeterminedproportions by weight, a typical composition of the refractory materialof the model 12 to which the base layer is applied, being as follows:

Material Weight % Silica (SiO₂) 20-88  Silicon Carbide (SiC) 2-40 Zircon(ZrSiO₄) 0-20 Aluminium Oxide (Al₂O₃) 0-20 Magnesium Oxide (MgO) 5-20Mono Ammonium Phosphate 5-20

The above materials are mixed to form a homogenous powder and thensealed in packets of moisture resistant material. The homogenous powder,when required, is then mixed with a colloidal silica based solution in avacuum mixer to form a creamy mixture which is poured into a siliconebased impression of the prepared tooth and allowed to set, to yield andactively obtain the desired set expansion and green strength. The silica(SiO₂) content can be varied in nature, i.e. fused silica, quartz,cristobalite or tridymite in order to alter the coefficient of thermalexpansion of the ultimate refractory material to suit use with low orhigh coefficient of thermal expansion dental dental porcelains. Theaddition of Silicon Carbide to the refractory material increases thethermal conductivity. Variation in the formulation and in the type andconcentration of the colloidal silica based solution will alter the setexpansion and green strength. Variation in the particle size of theconstituents will affect the density and smoothness of the finalrefractory.

After formation of the model 12 of refractory material, the model 12 issubjected to burn out and optionally a hardening heat treatment in afurnace at temperatures in the range 1000-1150° C. which, due to thesubsequent associated shrinkage of the model, ensures that a net overallexpansion to shrinkage may be designed in for different applications,i.e. for different restorations. For example, in the case of arefractory model for forming base layers for crown type applications, anet overall positive expansion is designed in, such that no ‘over’layer, i.e. a spacing/varnish/release layer, has to be applied to therefractory model 12 prior to the formation of the base layer as is thecase, for example, in prior proposals.

The refractory model 12 is cooled and during such cooling, i.e.simultaneously therewith, the base layer 4 is applied directly to thesaid model 12 by flame spraying techniques which include plasma sprayingand detonation gun methods of application, the base layer then beingsintered at a high temperature. The layers 6, 8, and 10 of dentalporcelains are then applied to said base layer.

The base layer 4 is sintered whilst it is still on the refractory model12—or alternatively it may be sintered following removal of therefractory model 12—by, for example, microblasting with glass beads.Similarly, the layers 6, 8, and 10 of dental porcelains may be appliedto the base layer either whilst the base layer 4 is still on therefractory model 12 or alternatively after removal of the refractorymodel 12.

Once the base layer 4 has been sintered—either on or off the refractorymodel 12, and in either case the model 12 removed—the base layer 4 maybe trimmed using a hand piece and diamond burr.

Referring now to FIG. 1a—where like parts are given the same referencenumerals as in FIG. 1 but with the suffix ‘A’—the base layer 4A may thenbe re-fitted to another refractory model 12A (formed in the same manner.as described above, and then heat treated) and the layers 6A, 8A, and10A of dental porcelains applied to complete the restoration in thenormal manner. In FIG. 1a, layer X represents a marginal material‘cerabond’ which yields extremely accurate margins.

Referring now to FIG. 2, the dental restoration 2—with the refractorymodel 12 removed—is shown applied to a natural tooth 14 through theintermediary of a layer 16 of bonding agent. Prior to the application ofthe dental restoration to the natural tooth 14, treatment of the toothsurface 18 may be necessary—depending upon the type of bonding materialused—such treatment involving the use of the phosphoric acid etchingtechnique. FIG. 2a shows the restoration of FIG. 1a applied to a naturaltooth 14A, and therefore like parts have been given the same referencenumerals as in FIG. 2 but with the suffix ‘A’.

The flame spray process used in the method of the invention utilises afree flowing powder or liquid which incorporates a technical ceramicbased material such as Alumina, Zirconia, or Titania, or combinationsthereof. The free flowing powder or liquid is introduced into theflame/plasma whereupon it is given kinetic energy and thermal energy anddirected at the target, i.e. the model 12 of refractory material whichwill have been pre-formed by the afore-mentioned process.

Swift passes are made so as to deposit the technical ceramic basedmaterial onto the model 12 of refractory material in a series ofsuccessive layers, this action serving to facilitate the fabrication ofa relatively dense microstructure.

Small amounts of silicate based materials may be added to the pre-cursorfeed materials to facilitate the production of a fully dense technicalceramic based material layer 4. Alternatively, the silicate basedmaterials may be added once the base layer has been formed.

It should be noted that, dependent upon the type of technical ceramicbased material used and the end result that is required, post-flamespray heat treatment of the base layer is necessary prior to theapplication of the layers of dental porcelain, not only to fully densifythe base layer but also to homogenise alloying additions or phasespresent and to enhance the optical properties of the base layer. Itshould be noted that it is only after trimming that the base layer 4 ofFIG. 1 becomes base layer 4A of FIG. 1a.

Due to the nature of the microstructure produced following flamespraying, any small amount of remaining shrinkage of the layer ispromoted throughout the depth of the layer, thus not affecting the fitof the final restoration to the tooth.

Utilising the optimum conditions outlined, a base layer approachingtheoretical density can be produced. The finished thickness of the baselayer will be dependent upon which type of restoration is beingmanufactured and may vary from a single particulate layer in the orderof 1 micro-meter to a substantial layer in the order of 3 millimeters.

The flame spray process used to produce the base layer is a ‘direct’technique—utilising the refractory model 12—and following the formationof the base layer, conventional techniques are used for the applicationof the layer(s) of dental porcelain. The first layer of dental porcelainmust have a co-efficient of thermal expansion which is matched to, orpreferably lower than that of the technical ceramic based material baselayer. A porcelain having this slightly lower co-efficient of thermalexpansion will be placed into slight compression upon cooling, thusyielding a dental restoration with optimum strength properties andaesthetics.

A dental restoration incorporating a base layer produced in accordancewith the invention—and formed on a model of refractory material asdefined and as referred to above—has excellent shape retention, sincethe normally large shrinkages (typically 15 to 20%) experienced onsintering a technical ceramic formed by casting, die pressing or slurrybuild-up techniques are greatly reduced or overcome. The strength of thedental restoration produced is greater than similar dental restorationswhich are manufactured from porcelain based materials.

The formation of the model of refractory material—of high thermalconductivity and matched thermal expansion—allows the base layer to beflame sprayed directly on to the cooled refractory model withoutproducing cracks in the base layer, unlike all commercially availablephosphate bonded dental refractories (having lower thermal conductivity)which have been found to be unsuitable for direct spraying, yieldingcracks in base layers and proving difficult, if not impossible, to blastout following further heat treatment. Thus more consistent, andcrack-free technical ceramic base layers can be produced.

It is thus the combination of a refractory of high thermal conductivity(incorporating SiC) with the cooling effect which confers the beneficialproperties on the base layer with respect to crack-free properties,unlike a previously proposed manufacturing process which was limited intime of processing so as not to overheat the model or support. It is thecombination of the formulation of the refractory with the formulation ofthe colloidal silica which confers the beneficial ‘active’ eliminableproperties on the refractory, thereby negating the need for an ‘over’layer, thus resulting in accurate fitting base layers.

The model of refractory material allows flame sprayed base layers to besintered at high temperatures (1000° C. to 1200° C.) on the refractorymaterial if required, and the refractory to be removed by microblastingwith glass beads in the normal manner either directly following sprayingor following sintering, or following porcelain build-up. Alternatively,the base layer may be sintered off the refractory model. Allcommercially available phosphate bonded dental refractories have beenfound to be too hard after such processing and cannot be removed bymicro-blasting. Additionally, use of cooling is not as effective due tothe low thermal conductivity of commercially available dentalrefractories.

The technique of removing the refractory, sintering and trimming thebase layer, and re-fitting the base layer to another refractory modelprior to the application of the layer(s) of porcelain, has the advantagethat the applied layer(s) of porcelain are supported during firing atthe marginal area and hence an accurate marginal fit may be produced infully fired, dense porcelain layer X which yields opticalbiocompatability with gingival tissue. Normally, base layers of anytype—whether ceramic or metal—have margins which are fired unsupportedand thus require special powders or liquids or combinations of the two.

The refractory model may have a net overall zero to small positiveexpansion actively designed in to yield extremely accurate margins.Further, this type of refractory is optionally hardened via a heattreatment cycle following burn out.

The formation of the refractory model 12A upon which a trimmed sinteredbase layer is fitted and subsequent layer(s) of porcelain applied tocomplete the final restoration is innovative.

There are various model systems including but not limited to Pindex, E-Ztray, Accutrak type, Single cast type etc., for which a refractory modelcan be produced to substitute into the prepared master model site.

It should be noted however that whichever model system is used, thefitting of a trimmed, formed (sintered) base layer to the refractorymodel, prior to the addition of layer(s) of porcelain is novel, andyields marginal areas of exact fit which are excellent in aesthetics andgingival biocompatibility.

Normally, porcelains, when used alone for veneer, inlay, onlay, andbridge type applications, need to be fired on the refractory. Withcommercially available low thermal conductivity refractory materials,different firing conditions are required (often higher temperature), butdue to the higher thermal conductivity of the refractory material usedin the method of the invention, the same or similar conditions andmaterials can be used for firing in dental furnaces on the model ofrefractory material.

In addition, and with special reference to low fusing porcelains, theporcelains may be fired reproducibly at any point in the furnace muffle,i.e. either centrally or radially adjacent to the heating elements.

Finally, by a combination of materials and process variations,satisfactory aesthetics can be produced which take account of hue,value, chroma, translucency, shape, outline form, contour, proportionand soft tissue harmony with the oral cavity.

What is claimed is:
 1. A method for the production of a dentalrestoration comprising the steps of: (a) forming a model of refractorymaterial of a tooth, said refractory material consisting essentially ofan admixture of Silica, Silicon Carbide, Magnesium Oxide, and MonoAmmonium Phosphate in predetermined proportions by weight, andoptionally Zircon or Aluminium Oxide in predetermined proportions byweight; (b) forming by flame spraying directly onto said refractorymodel a base layer of technical ceramic of predetermined thickness ofsaid restoration, said refractory model being of high thermalconductivity and cooled during the formation of said base layer; (c)removing the refractory model immediately after the formation of saidbase layer; (d) sintering said base layer of technical ceramic basematerial; and (e) applying to said base layer one or more layers ofdental porcelain.
 2. A method according to claim 1, including theadditional step of subjecting the refractory model to burn out attemperatures in the range of 1000-1150° C.
 3. A method according toclaim 1, including the further step of subjecting the refractory modelto a hardening heat treatment at temperatures between 1000-1150° C.
 4. Amethod according to claim 1, including the further step of addingsilicate based materials either into the precursor powder or onto saidbase layer following its formation and prior to sintering.
 5. A methodaccording to claim 1, wherein the refractory model is removed bymicroblasting with glass beads.
 6. A method for the production of adental restoration comprising the steps of: (a) forming a model ofrefractory material of a tooth, said refractory material consistingessentially of an admixture of Silica, Silicon Carbide, Magnesium Oxideand Mono Ammonium Phosphate in predetermined proportions by weight; (b)forming by flame spraying directly onto said refractory model a baselayer of technical ceramic material of a predetermined thickness of saidrestoration, said refractory model being of high thermal conductivityand cooled during the formation of said base layer; (c) sintering saidbase layer of technical ceramic based material; (d) removing therefractory model after the sintering recited in Step (c); and (e)applying to said base layer one or more layers of dental porcelain.
 7. Amethod according to claim 6, wherein the removing of the refractorymodel recited in Step (d) is performed by microblasting with glassbeads.
 8. A method according to claim 6, wherein the forming of therefractory model recited in Step (a) includes, subjecting the refractorymodel to burn out at temperatures in the range of 1000-1150° C.
 9. Amethod according to claim 7, further including subjecting the refractorymodel to a hardening heat treatment at temperatures between 1000-1150°C.
 10. A method according to claim 6, including one of (i) addingsilicate based materials into a precursor powder used in the forming ofsaid base layer recited in Step (b), and (ii) adding silicate basedmaterials after said base layer is formed in Step (b).
 11. The methodaccording to claim 1, including the step of applying said base layer oftechnical ceramic based material to a further refractory model.
 12. Themethod according to claim 6, including the step of applying said baselayer of technical ceramic material to a further refractory model.